Thermal Interface Materials

The heat generated by individual power devices, integrated circuits, and within complete electronic systems has increased radically in the last decade. The need for thermal management systems is essential to preserve electronic systems operating within their specification. Peak performance and reliability depend on proper temperature limits sustained by thermal management systems.

When surfaces are attached with an interface, there is most commonly an area of mechanical contact at the interface sight. This is due to the surface roughness or waviness and will have an impact on the heat conduction. The surface irregularity and the resulting gap is the primary cause of thermal contact resistance as the gaps fill with low thermal conductivity air. To minimize the resistance, filler materials are generally required to increase the contact between the mating surfaces.

The importance of minimizing contact resistance requires the use of thermal interface materials (TIMs). TIMs go into the joint to fill the air gaps between solid surfaces during assembly. Typically, several interfaces exist between the heat generating element and the eventual heatsink. The thickness can vary from a few thousandths of an inch to several hundredths of an inch.

Some of these consist of permanent bonds like solder or adhesives. Other interfaces are non-permanent and will form part of the heat transfer path, such as a component being bolted to a heatsink or between an assembled module and a chassis.

KraFAB offers a wide variety of TIMs that are used in electronic devices to assist in transferring heat away from components. The different designs for devices often required a variety of options to transfer heat.

Some of the thermal interface materials used to transfer heat from electronic devices includes:

Thermal tapes

Phase change materials

Gap pads

Thermal greases

Dielectric pads

Heat spreaders

Thermal compounds

Thermal Gel

The ideal TIMs have high thermal conductivity, are easily deformed by small contact pressure, have no leakage from the interface, no deterioration over time, are non-toxic and are easy to apply or remove.

KraFAB’s thermally conductive materials are used to reduce air gaps from electronic devices by conforming uneven rough mating surfaces. As electronic devices get smaller, conductive materials need to respond and work efficiently to ensure the devices are working at maximum capacity. KraFAB has a wide variety of materials that is well suited for your design.

Many electronic devices use fans and heat sinks to keep the temperatures cool, but thermal interface materials play a critical role in assisting with the displacement of heat.

KraFAB offers a wide variety of fabricated films, wet dispensed thermal materials and pad thermal interface materials to fit your application needs. Wet dispensed materials such as gel, adhesives and non-curing compounds can be used in practically any device configuration and do an excellent job in accommodating high tolerance between surfaces.

We can meet the demands of your electronic designs

TIM Considerations

While the use of thermal interface materials helps to improve heat transfer across an interface, TIMs also account for most of the total system thermal resistance. When choosing a thermal interface material, many characteristics must be considered:

Thermal conductivity

A thermal interface material’s conductivity determines how much heat it can transfer across the interface. This has a major impact on its thermal performance.

Thermal resistance

The sum of resistance at the interface is due to the thermal conductivity and the contact resistance of the two surfaces.

Electrical conductivity

Some TIMs are electrically conductive but they are generally polymers or polymers filled with non-conductive materials.

Ease of Application and Installation

The ease of application of a thermal interface material is based on the material and the controlled amount applied.

Phase Change Temperature

This is the temperature at which the interface material transitions softens in order to fill the gaps and to expel all air. It is important that the phase change temperature is below the maximum operating temperature so that heat can be transferred across the interface. The temperature also needs to be as high the device can tolerate, however, to avoid a phase change during shipping.

Outgassing

The release of volatile gasses when materials are exposed to elevated temperatures and/or low atmospheric pressures is outgassing. Outgassing is a particular concern in aerospace applications due to reduced pressures and can also cause issues within sealed cavity packages.

Surface Finish

The efficiency of a TIM to fill large gaps in irregular surfaces is an important factor in material choice. The interaction of filler particles with the adjoining surfaces influences the level of compaction at the interfaces.

Pressure

A significant difference in the thermal interface material used is based on mounting pressure. Minimizing contact resistance requires considering TIM performance and the ability to conform to surfaces.

Mechanical Properties

TIMs in paste or liquid needs to be dispensed or printed. A higher filler volume fraction increases thermal conductivity but also makes it more difficult to dispense due to the increased viscosity.

Long Term Stability & Reliability

TIMs need to perform consistently throughout the lifespan of the device. Electronic devices are designed to last seven to ten years, while avionics and telecommunication devices can survive decades.

Types, Disadvantages, & Advantages of Traditional TIMs

Thermal grease, phase change materials, and filled polymers are the traditional thermal interface materials used most commonly in applications.

Thermal grease

A good TIM choice for its high thermal conductivity, low thermal resistance, and low cost. Thermal grease also does not require curing, and delamination is not an issue. The disadvantages of thermal grease is that its thermal cycling can result in pump-out and phase separation. The reliability of thermal grease can also be reduced if it dries out. Thermal grease is difficult to control due to its thickness, and excess grease can be hard to contain.

Filled Polymers

Offer an easy-to-handle, clean, no pump-out application. They also resist harsh environments and can be easily cut to size. Polymers, however, require curing and do not flow freely. They are also more expensive and needs permanent clamping.

Phase change materials

This includes low melting alloys and polymerics. Low melting alloys require no curing and are easy to apply. They can dry-out, however, and can oxidize and/or corrode at elevated temperatures. Polymerics do not dry-out, delaminate, or require cure. They also have an easier application process than grease, as well increased stability and less pump-out. The disadvantage in using polymerics is that they require constant pressure, and have a lower thermal conductivity and a greater surface resistance than thermal greases.

Due to the combination of a decrease in the size of electronics and an increase of power densities, thermal control in electronics have become a major concern. In order for electronics to properly operate, thermal management is becoming more critical and fundamental. Maintaining temperatures within their proper limits ensures optimum performance and reliability.

Thermal management systems need to not only use all modes of heat transference but also to spread the heat out from the point of generation and into extended surface areas. This requires a knowledge of traditional thermal interface materials so that the best material can be chosen for each application.